The engine control module (ECM) uses this diagnostic to determine if the engine coolant temperature (ECT) has reached the minimum calibrated thermostat regulating temperature. The ECM supplies 5 volts to the ECT signal circuit , and a ground for the ECT low reference circuit. The ECM monitors the temperature of the coolant by using the input received from the ECT sensor. The ECM calculates the amount of fuel burned since start-up to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally, or does not reach the regulating temperature of the thermostat, the engine is considered not warm enough for stable, low emission operation.

The purpose of both these rationality diagnostics is to use the ECT sensor to determine if the engine coolant will heat up at the correct rate, and also meet the calibrated target temperatures under various operating conditions.

The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation.

The engine did not meet the ECT target temperature of 45°C (113°F) or 72°C (162°F) during expected calibrated warm-up times and start-up temperatures.

DTC P0128 is designed to detect for a stuck open thermostat.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Electrical Information Reference

Scan Tool Reference

Caution: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.

Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.

1. Verify the proper heat range and for a stuck open thermostat. Refer to Thermostat Diagnosis .
2. Inspect the ECT sensor terminals for corrosion and for engine coolant leaking through the sensor.
3. Verify the proper operation of the engine cooling fan, fan stuck ON/fully engaged all the time.
4. Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
5. Engine running, observe the DTC information with a scan tool. DTC P0128 should not set.
6. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Verify the proper heat range and for a stuck open thermostat. Refer to Thermostat Diagnosis .
2. Ignition OFF, disconnect the harness connector at the ECT sensor.
3. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal A and ground.

⇒	If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the control module.

4. Ignition ON, verify the scan tool ECT parameter is less than -39°C (-40°F).

⇒	If greater than the specified range, test the signal circuit terminal B for a short to ground. If the circuit tests normal, replace the control module.

5. Install a 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT parameter is greater than 149°C (300°F).

⇒	If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the control module.

6. If all circuits test normal, test or replace the ECT sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.